Today, in North America and around the world, the vast majority of railway crossties used are made of rectangular pieces of creosote-impregnated hardwood or softwood. Typical dimensions for these ties are about 2.4-2.6 m in length, about 18-23 cm in width, and about 15-18 cm in height. Even ties made from newer materials, such as concrete, steel, plastic or composites in general, mimic this standard rectangular design.
One of the key railway safety and performance issues is lateral track stability i.e. sideways movement of the ties perpendicular to the parallel rails, particularly, the ability of the entire track to resist lateral movement when subjected to forces produced by the movement of trains. Lateral track movement is costly because it requires expensive maintenance to reposition the track and, if left to exceed established limits, the lateral travel movement can cause track failure and, ultimately, train derailments.
Typically when installed, the ties are partially buried in rock particles known as ballast. The crosstie/ballast relationship is one of the main defenses against lateral track movement. The better the tie is “locked” into the ballast, the more the track will resist lateral movement. The rail industry relies on friction between the ballast and wooden ties to resist movement. This friction increases over time due to the formation of pits or pockets in the ties caused by abrasion from the rock particles. Unfortunately, in situations where significant lengths of track have been replaced, this “conditioning phase” can force the track user to temporarily slow train movements, which in turn can cause operational complexity and loss of efficiency. This is one of the reasons that steel ties have not found wide spread acceptance. Their friction factor is very low and, consequently, they are susceptible to longitudinal movement, i.e. movement parallel to the railway lines. Extruded HDPE (high-density polyethylene) tie producers have tried to deal with HDPE's natural “slipperiness” by embossing or pitting the side surfaces, while respecting the original tie dimensions.
U.S. Pat. No. 4,285,115, issued Aug. 25, 1981 to Arbed, describes a steel or concrete sleeper having improved directional stability and sliding resistance due to the sleeper being formed in a Y-shape. Each arm of the Y-shape is of identical and consistent cross-section.
U.S. Pat. No. 6,230,981 B1—issued May 15, 2001 to Corus U.K. Limited describes a steel railroad sleeper (tie) of inverted channel section that may have a waisted central section of reduced width. U.S. Pat. No. 6,230,981 B1 focuses exclusively on producing a hollow sleeper from cold rolled steel. The purpose of the reduced central cross-section is to create greater locking of ballast that must be propelled into the sleeper interior and to reduce the amount of ballast necessary to fill the sleeper interior. The central region can also be filled with foam that would actually prevent ballast from entering this region. It is not clear whether the waisted central section is for locking the sleeper into position or to increase friction between the tie and the ballast.
United States Patent Application No. 2003/0085293 A1, published May 8, 2003 to Nosker et al, describes a crosstie surface design consisting of a pattern of indentations that contacts the ballast, which increases the ties' resistance to sliding (abstract). It is an alternative pattern to previous attempts at surface scoring (paragraph 0020). It considers molding or embossing the pattern into the tie so as not to compromise the said ties' overall rectangular cross-sectional dimensions (paragraph 0024).
However, none of the aforesaid prior art references provides ties having sufficiently improved efficacy in preventing lateral tie slippage and longitudinal track movement with reduced tie volume and, thus, material costs.
There is, therefore, a need for an improved tie that decreases the lateral movement of the tie within the ballast, while at the same time minimize the volume of the tie and, thus, the amount of material required.